1. Field of the Invention
The present invention relates to improvements in a speed estimation observer which is applied to a speed control system using a rotary encoder and operated in an extremely low speed range.
2. Description of the Background Art
Generally, in a speed control system for controlling a speed of a motor by using a rotary encoder of a relatively low resolution, when the motor is rotated in an extremely low speed, an encoder pulse interval becomes longer than a speed control period, and therefore accurate speed information cannot be obtained during such a speed control period. Such a relationship between the speed control period and encoder pulse is shown in FIG. 9. In FIG. 9, Ts is a speed control period, Tp is a period of a pulse encoder, and Td is a difference between Ts and Tp. Accordingly, when the encoder pulse is inputted, an average value of a speed is derived by using; a pulse period Tp from the following equation: EQU n.sub.M =60/pp.multidot.1/T.sub.p
where pp is the number of pulses per rotation of the encoder [P/R], Tp is an encoder pulse period [sec], and n.sub.M is an average value of the rotation speed of the motor [rpm].
As is clear from the above equation, in an extremely low speed range, that is, when the encoder pulse period becomes extremely large, the average speed becomes indefinite. Accordingly, the speed control system tends to be unstable, and responsibility thereof is degraded. In order to solve such difficulties, there is proposed a control system in which a speed during an interval between adjacent encoder pulses is estimated by using a speed estimation observer. Such a control system is shown in FIG. 11. A speed estimation observer 10 applied to the control system is shown in FIG. 10 and includes a load torque observer block of a minimum dimension. As shown in FIG. 10, a deviating block 110 receives a torque command .tau..sub.M *.sub.(i) and a load torque estimate .sub.(j), and outputs a deviation output therebetween. The deviation output is supplied to a first calculating block 120. The first calculating block 120 includes a division block 120a in which the speed control period Ts is divided by a model machine time constant T.sub.M *, an integrator 120c, and an adder 120b which adds an output of the division block 120a to an output of the integrator 120c. The model output speed estimate '.sub.(i) derived in the first calculating block 120 is supplied to a second calculating block 130. The second calculating block 130 derives an average value during pulse interval and outputs the calculated result '.sub.(j) to a plus input end of a first deviating block 140. A minus input end of the first deviating block 140 receives an average value n.sub.M .sub. (j) which is of a speed detection output from the pulse encoder 15. The deviation output of the first deviating block 140 is supplied to an observer gain block 160 and multiplied by a predetermined (generally proportional) gain. The multiplied value is outputted as a load torque estimate .sub.(j) to the deviating block 110. In addition, the deviation output of the first deviating block 140 is supplied to a minus input end of a second deviating block 170. A pulse input end of the second deviating block 170 receives the model output estimate .sub.(j). The second deviating block 170 outputs the speed estimate .sub.(i). Since in the speed estimation observer shown in FIG. 10 the speed estimate .sub.(i) and the load torque estimate .sub.(j) are simultaneously obtained, a disturbance compensation is carried out for obtaining a disturbance suppressing effect. As shown in FIG. 11, the speed estimation observer block 10 outputs a speed estimate .sub.(i) and a load torque estimate .sub.(j). The speed estimate .sub.(i) and the speed set value .tau..sub.M *.sub.(i) are supplied to a minus input end and a plus input end of a third deviating block 18, respectively. The deviation output therefrom is supplied to a speed amplifier 19 of a proportion gain K.sub.WC. An adder 20 adds the output from the speed amplifier 19 and the load torque estimate .sub.(j), and outputs a torque command .tau..sub.L *. A fourth deviating block 21 receives the torque command .tau..sub.L * and the actual load torque .tau..sub.L, and outputs a deviation therebetween. The deviation is supplied to a motor 22 for its control. A speed detection block 23 receives a signal from a pulse encoder 15 and outputs the averaged value to the speed estimation observer 10.
The manner of operation of the speed estimation observer 10 applied to a motor speed control system shown in FIGS. 10 and 11 will be discussed.
A model output estimate .sub.(i) is obtained in a manner that the deviation between the torque command .tau..sub.M *.sub.(i) and the load torque estimate .sub.(j) is integrated with respect to the observer model machine time constant T.sub.M *. Following this, an average speed value .sub.(j) during each pulse interval Tp is obtained from the model speed output estimate '.sub.(i), and a deviation between the average speed value n.sub.M .sub.(j) and the average speed value '.sub.(j) is obtained. Multiplying the deviation by the observer gain (g), the load torque estimate .sub.(j) is obtained. Then, the speed estimate .sub.(i) is derived by subtracting the deviation at the first deviating block 140 from the observer model output estimate .sub.(i). The derived speed estimate .sub.(i) is supplied to the speed control amplifier 19 for executing the control of the motor 22. Also, it becomes possible to execute a load disturbance compensation by obtaining the torque command upon adding the load torque estimate .sub.(j) and the output of the speed amplifier 19 in the adder 20.
However, if the observer gain is set at a relatively large value for obtaining the disturbance suppressing effect in a condition that an encoder pulse interval is smaller than a speed control period, such a large observer gain tends to invite a unstableness of the speed control system in a condition that the encoder pulse interval is longer than the speed control period. Therefore, the observer gain should not be set at a large value. However, on the other hand, if the observer gain is set at a relatively small value, the disturbance suppressing effect in a high motor speed range may be degraded.